ibmvscsis: Fix sleeping in interrupt context

Currently, dma_alloc_coherent is being called with a GFP_KERNEL
flag which allows it to sleep in an interrupt context, need to
change to GFP_ATOMIC.

Cc: stable@vger.kernel.org
Tested-by: Steven Royer <seroyer@linux.vnet.ibm.com>
Reviewed-by: Michael Cyr <mikecyr@linux.vnet.ibm.com>
Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>